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The ants go marching -- and manage to avoid traffic jams

Posted April 2, 2003; 02:37 p.m.

by etu

Princeton biologist
Iain Couzin
has found a society in which hundreds of thousands of travelers speed along densely packed roadways, transporting huge amounts of materials, all without a bit of congestion.

If they don't sound like the kind of drivers you're used to dealing with, they aren't. Couzin, a postdoctoral researcher in
ecology and evolutionary biology
, found this paragon of efficiency in colonies of army ants in Panama.

In a recently published study, Couzin and Nigel Franks of the University of Bristol in the United Kingdom, showed that these ants have evolved a three-lane traffic system that puts snarled human highways to shame. The traffic system achieves the maximum possible flow as 200,000 ants pour out of their nest on giant hunting raids and return with more than 30,000 pieces of slain prey in a day.

The study, a unique combination of mathematical analysis and field observations, is part of a broad effort to understand the highly coordinated movements of all sorts of organisms, whether flocks of birds, schools of fish or even groups of single cells. "The collective properties of these groups are really just stunning," said Couzin.

In the case of the army ants, scientists had previously observed that ants returning to the nest, with their booty of slain grasshoppers and other insects, form a center lane, while outgoing ants split into lanes on either side. The practical benefits of this arrangement and the behaviors that give rise to it, however, were completely unknown.

In their paper, published in the Jan. 22 issue of the Proceedings of the Royal Society of London, Series B, Couzin and Franks found that a key factor is the angle at which the ants turn aside when they collide with oncoming nestmates. Outbound ants tend to turn relatively sharply, while returning ants stay on a straighter line, thereby cutting a center path between two outgoing lanes. This arrangement seems to make sense, since incoming ants are likely to be less maneuverable due to their heavy cargoes, said Couzin.

Ants stick to this pattern, however, even when the incoming ones have nothing to carry. The reason, concluded Couzin, is that it is simply an efficient way to travel.